1. Field of the Invention
The present invention relates to a method of producing a solar cell; a solar cell, and a method of producing a semiconductor device. In particular, it relates to a method of producing a silicon solar cell which has electrical separation between the p-doped layer and the n-doped layer of a p n junction necessary for easy formation of the electrodes with a high productivity; a solar cell having the p-doped layer and the n-doped layer of a p n junction which are electrically separated in a simple manner, and a method of producing a semiconductor device in which the electrode capable of substantially reducing the contact resistance to the semiconductor substrate by applying the same physical phenomenon used in the electrical separation of the p n junction in forming the electrodes of the solar cell.
2. Background of the Invention
Currently, silicon solar cells are the most common power solar cells. In general, the process for large scale production needs to be simplified in order to reduce the associated manufacturing costs.
The following description of a conventional manufacturing process refers to FIGS. 17a-f which is a diagram of the manufacturing steps of a solar cell.
FIG. 17a depicts a p type Si substrate 1. In FIG. 17b, phosphorus (P), for example, can be thermally diffused into the p type Si substrate to form an n type diffusion layer 2 by inverting the electric conduction type.
Phosphorus oxychloride (POCl.sub.3) is often used as a diffusion source of phosphorus. Further, when there is no special structure, the n type diffusion layer is formed in the entire surface of the p type Si substrate 1. The sheet resistance of the n type diffusion layer is about several tens .OMEGA./.quadrature. and the depth of the diffusion layer is about 0.3-0.5 .mu.m.
The n type diffusion layer 2 has applied thereon a resist on each of the surfaces except for the principal plane (light incident plane), and an etching process is performed such that the n type diffusion layer 2 remains only in the principal plane (FIG. 17c). The resist is removed with an organic solvent.
Then, screen printing is performed with an aluminum paste 3 and is conducted on the surface opposite to that in which the n type diffusion layer is formed, and the substrate is baked in a near-infrared furnace at 700.degree. C.-900.degree. C. for several min to ten and several min. As a result, aluminum is diffused as an impurity from the aluminum paste into the p-type Si substrate 1 whereby a Back Surface Field (BSF) layer 4 as a p.sup.+ layer having a high concentration of the impurity is formed as shown in FIG. 17d. The energy conversion efficiency of the solar cell is attributed to the BSF layer 4.
Then, a comb-shaped pattern of silver paste is screen-printed to form an n electrode 5 on the n type diffusion layer 2 on the principal plane as shown in FIG. 17e. The substrate with the n type diffusion layer 2 and the n electrode 5 is baked at 700.degree. C.-800.degree. C. whereby a solar cell is completed.
For simplifying the present disclosure, specific illustration has been omitted of many cases wherein an antireflection film such as TiO.sub.2, SiN, SiO.sub.2 or the like has been formed on the n type diffusion layer 2 in order to improve the energy conversion efficiency.
FIG. 17f shows a structure of a solar cell produced according to the process similar to the above-mentioned process wherein the n type diffusion layer 2 remains on the side surfaces of the p type Si substrate 1 in addition to the principal plane. In this case, the portion indicated by reference numeral 6 has a p.sup.+ /n junction. The concentration of the impurity (phosphorus) at or near the surface of the n type diffusion layer 2 is very high on the order of 10.sup.20 -10.sup.21 cm.sup.-3, and such a structure is apt to have electrical leakage. As a result of leakage, the energy conversion efficiency of the solar cell is remarkably reduced. Namely, if the n type diffusion layer is not formed in just the single plane (light incident plane) as shown in FIG. 17c, a solar cell having excellent characteristics can not be manufactured. In the conventional technique, as described above, the application of a resist to protect the surface of the diffusion layer, the etching treatment and the removal of the resist are required. In order to reduce the manufacturing cost, however, it is necessary to simplify these processes as much as possible.